Which statement about the distribution of alveolar ventilation in upright lungs is true?

Which statement about the distribution of alveolar ventilation in upright lungs is true?

• A. Alveoli at the base of the lungs (dependent alveoli) are better ventilated than those at the apex
• B. Alveoli at the apex of the lungs (nondependent alveoli) are better ventilated than those at the base
• C. All areas of the lungs are ventilated equally
• D. Alveoli at the central regions of the lungs are better ventilated than those at the base or apex

Answer: (A)

In upright lungs, gravity creates a vertical gradient in pleural pressure, which shifts how much each region can change volume during a breath. The base (dependent region) ends up with less inflated alveoli at end expiration and, on the inspiration that follows, has a greater capacity to expand because those alveoli are on a more favorable part of the pressure–volume curve. The apex, being more expanded at rest due to more negative pleural pressure, sits on the flatter portion of the curve, so its tidal-volume–driven expansion is smaller. As a result, the dependent alveoli at the base receive more of the tidal breath than those at the apex, making base ventilation greater. This pattern also aligns with gravity-driven perfusion favoring the base, optimizing overall ventilation–perfusion matching.

In upright lungs, gravity creates a vertical gradient in pleural pressure, which shifts how much each region can change volume during a breath. The base (dependent region) ends up with less inflated alveoli at end expiration and, on the inspiration that follows, has a greater capacity to expand because those alveoli are on a more favorable part of the pressure–volume curve. The apex, being more expanded at rest due to more negative pleural pressure, sits on the flatter portion of the curve, so its tidal-volume–driven expansion is smaller. As a result, the dependent alveoli at the base receive more of the tidal breath than those at the apex, making base ventilation greater. This pattern also aligns with gravity-driven perfusion favoring the base, optimizing overall ventilation–perfusion matching.